Wavelength division multiplexing (“WDM”) is a technique for transmitting separate wavelengths of light through the same optical fiber. WDM can be accomplished by first assigning an optical transmitter to each wavelength generated by one or more light sources. Each optical transmitter receives a different electronic digital signal and modulates that signal onto the assigned wavelength. A modulated wavelength is called an “optical signal” or “optical channel.” Separate fibers each carry one optical signal to an optical multiplexer that combines the optical signals for transmission over the same optical fiber. If the system is designed properly, the separate optical signals do not interfere with each other while being transmitted over the same optical fiber. On the hand, a demultiplexer is a device that receives the optical signals carried by the same optical fiber and separates the optical signals for transmission on separate optical fibers or waveguides.
Arrayed waveguide gratings (“AWGs”) are commonly used as optical multiplexers or demultiplexers. AWGs are capable of multiplexing a large number of wavelengths into the same optical fiber and demultiplexing a large number of optical channels carried by the same optical fiber into separate optical fibers. For demultiplexing, the optical signals are input from the same optical fiber into a first multimode section, the optical signals propagate along several single-mode waveguides to a second multimode section, and finally into output fibers. Wavelength separation is based on an interference effect and the different optical path lengths of the single-mode waveguides. The result is that each of the output fibers carries a different optical signal. Multiplexing can be accomplished by operating the AWG in reverse. In other words, each optical signal can be input to the second multimode section on a separate optical fiber, and multiplexed optical signals are output from the first multimode section on the same optical fiber.
However, typical multiplexers and demultiplexers, such as the AWG, are relatively large in size and lossy. Physicists and engineers continue to seek enhancements in multiplexer and demultiplexer design and performance.